1. Field of the Invention
The present invention relates to a process for the preparation of .omega.-lactams, in particular caprolactam, with improved yields.
The present invention further relates to the .omega.-lactams thus obtained.
2. Prior Art
It is known to prepare caprolactam by reaction of cycloaliphatic compounds with nitrosating agents in the presence of a dehydrating agent. In U.S. Pat. No. 3,356,675 the preparation of caprolactam is described starting from precursors which contain the cycloaliphatic ring, in particular a cyclohexyl ring, by reaction of the same with nitrosyl acid sulphate in concentrated sulphuric acid medium and employing oleum as dehydrating agent. The reaction is carried out continuously and by stages at increasing reaction temperatures, wherein the contact times and the molecular ratios of the reagents must be predetermined.
In the aforementioned patent, the molecular ratios of the reagents, the temperature, and the reaction time, are established by preliminary tests carried out with the purpose of optimizing the process. In U.S. Pat. No. 3,356,675, when an SO.sub.3 /NOHSO.sub.4 molecular ratio between 0.6 and 0.7 to 1 is maintained, a caprolactam yield of about 92-94% is obtained, the purity of the caprolactam obtained being about 98%. When oleum is employed in the reaction of the formation of caprolactam from hexahydrobenzoic acid, a part of this latter is transformed by sulphonation on the carbon in .alpha. position with respect to the carboxyl group, to form cyclohexane-1-sulphone-1-carboxylic acid, whereby the reaction yields are decreased.
It is further difficult industrially to carry out the lactamization of the cycloaliphatic compounds in stages while maintaining different temperatures from stage to stage, as taught by U.S. Pat. No. 3,356,675. Said yields of the known processes, although they are rather high, involve the formation of considerable amounts of by-products which cannot be used and have to be discharged or disposed of in a different way, at a considerable expense. Therefore it is desirable to obtain possibly quantitative yields and therefore an ecologically improved process.